1. Background of the Invention
The present invention relates to a composition and a method for making an intergranular insulation type semiconductive ceramic capacitor comprising an insulating layer formed at grain boundaries defined among semiconductive crystal grains with a SrTiO.sub.3 based polycrystalline structure.
2. Description of the Prior Art
Such an intergranular insulation type semiconductive ceramic capacitor with a SrTiO.sub.3 based polycrystalline structure conventionally uses thin grain boundaries formed among its semiconductive crystal grains as a dielectric layer and thus exhibits a high dielectric constant, as compared with general ceramic capacitors made of a BaTiO.sub.3 based material. Since the ceramic capacitor is made of a material containing, as its main component, SrTiO.sub.3 which is a paraelectric material, it also exhibits a low dielectric loss (tan .delta.) at a room temperature and an improvement over the ceramic capacitors made of the BaTiO.sub.3 based material.
Such an intergranular insulation type semiconductive ceramic capacitor is manufactured by adding Nb.sub.2 O.sub.5, Dy.sub.2 O.sub.3, GeO.sub.2 and SiO.sub.2 as the secondary component to SrTiO.sub.3 as the main component, to produce a mixture, sintering the mixture in a reducing atmosphere at a low partial pressure of oxygen, to produce semiconductive crystal grains, and diffusing an insulating material such as PbO, Bi.sub.2 O.sub.3, B.sub.2 O.sub.3 and CuO into grain boundaries defined among crystal grains, to form an intergranular insulating layer.
However, this conventional method has disadvantages in workability and cost, because of requiring complex and troublesome steps such as the sintering step performed in the reducing atmosphere and the subsequently performed diffusing step for forming the intergranular insulating layer. Furthermore, the conventional method encounters a difficulty in manufacturing capacitors having a uniform characteristic, since the characteristic of a final capacitor product is dependant upon the conditions of the reducing atmosphere and the insulating layer.